This invention relates to the production of superplastically formed metal alloy panel structures, and is more particularly directed to novel procedure for the production of a superplastically formed and diffusion bonded trusscore structure, and to the resulting improved structure.
Superplasticity is the characteristic demonstrated by certain metals to develop unusually high tensile elongations with minimum necking when deformed within a limited temperature and strain rate range. This characteristic, peculiar to certain metal and metal alloys has been known in the art as applied to the production of complex shapes. It is further known that at these same superplastic forming temperatures the same materials can be diffusion bonded with the application of pressure at contacting surfaces.
Diffusion bonding refers to the solid-state, metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a time duration so as to effect intimate surface contact and cause comingling of atoms at the joint interface.
U. S. Pat. No. 4,304,821 to Hayase, et al discloses the structure and the process for making a metallic sandwich structure or box section panel in which metal worksheets, preferably made from titanium alloy, are joined in a preselected pattern by an intermittent or discontinuous weld. The joined sheets are sealed by a continuous weld to form an expandable envelope. Application of inert gas pressure to the envelope in a limiting fixture superplastically produces the sandwich structure as the expanded structure diffusion bonds to itself or other worksheets. Core configuration of the structure is determined by the weld pattern.
In some panel structure configurations, during superplastic forming of the core sheets the thinning is not consistent over the entire core sheet areas, some areas thinning out more quickly than other areas. Under these conditions when starting with a thick material, following superplastic forming, the resulting structure will have certain areas that are very thin and other areas that are very thick, resulting in a weight penalty where it is desired to form a completed structure in which the thin areas have a predetermined thickness. Also, in many instances for production of panel structures including trusscore structures, long and complex superplastic forming cycles are required, increasing the cost of production. Also, in some processes it is difficult to employ materials having relatively low super plasticity such as aluminum.
It is an object of the invention to provide an improved trusscore structure.
Another object of the present invention is to provide a novel process for producing a trusscore structure by superplastic forming and diffusion bonding.
Another object is the provision of procedure for producing a trusscore structure using simple and short forming cycles.
A further object is to provide a process of the above type, enabling the use of materials having less superplasticity such as aluminum.
Yet another object is the provision of a process for fabricating trusscore structures by superplastic forming so as to result in substantially uniform thinning over the superplastically formed areas of the structure.
Other objects and advantages will appear hereinafter.